title: Radioimmunoassay of "free thyroxin" in dried blood spots on filter paper - preliminary observations on the effective differentiation of subjects with congenital hypothyroidism from those with subnormal thyroxin-binding globulin and normal subjects.
abstract: In this sensitive, simple method for measuring "free thyroxin" (FT4) in eluates of dried blood spots on filter paper by use of a radioimmunoassay kit (Amerlex Free T4 RIA), the measurable range of FT4 is 1.8 to 57 ng/L (equivalent to the concentration in serum), or 7 to 237 fg/tube. The mean coefficients of variation for within assay-within spots, within assay-between spots, and between assays were 5.3%, 5.0%, and 6.2%, respectively. FT4 in blood spotted on filter paper is stable for at least a month when dried and kept at either -20 degrees C, 4 degrees C, room temperature (about 25 degrees C), or 37 degrees C. The results for FT4 in dried blood spots correlated closely with the free-T4 concentration in serum (r = 0.99). The method can be used to differentiate cases of primary and secondary hypothyroidism from normal subjects and those with subnormal thyroxin-binding globulin. This method may be useful in screening for congenital hypothyroidism, because sample-retesting is not necessary.
mesh_terms: D000293:Adolescent; D000328:Adult; D003409:Congenital Hypothyroidism; D005260:Female; D005374:Filtration; D006801:Humans; D007037:Hypothyroidism / Q000097:blood*; D007223:Infant; D007231:Infant, Newborn; D011863:Radioimmunoassay / Q000379:methods*; D012712:Serum Globulins / Q000032:analysis*; D013974:Thyroxine / Q000097:blood*; D013975:Thyroxine-Binding Proteins / Q000032:analysis*
chemical_list: D012712:Serum Globulins; D013975:Thyroxine-Binding Proteins; D013974:Thyroxine